Use delayed error handling for Encodable and Encoder infallible.

There are two impls of the `Encoder` trait: `opaque::Encoder` and
`opaque::FileEncoder`. The former encodes into memory and is infallible, the
latter writes to file and is fallible.

Currently, standard `Result`/`?`/`unwrap` error handling is used, but this is a
bit verbose and has non-trivial cost, which is annoying given how rare failures
are (especially in the infallible `opaque::Encoder` case).

This commit changes how `Encoder` fallibility is handled. All the `emit_*`
methods are now infallible. `opaque::Encoder` requires no great changes for
this. `opaque::FileEncoder` now implements a delayed error handling strategy.
If a failure occurs, it records this via the `res` field, and all subsequent
encoding operations are skipped if `res` indicates an error has occurred. Once
encoding is complete, the new `finish` method is called, which returns a
`Result`. In other words, there is now a single `Result`-producing method
instead of many of them.

This has very little effect on how any file errors are reported if
`opaque::FileEncoder` has any failures.

Much of this commit is boring mechanical changes, removing `Result` return
values and `?` or `unwrap` from expressions. The more interesting parts are as
follows.
- serialize.rs: The `Encoder` trait gains an `Ok` associated type. The
  `into_inner` method is changed into `finish`, which returns
  `Result<Vec<u8>, !>`.
- opaque.rs: The `FileEncoder` adopts the delayed error handling
  strategy. Its `Ok` type is a `usize`, returning the number of bytes
  written, replacing previous uses of `FileEncoder::position`.
- Various methods that take an encoder now consume it, rather than being
  passed a mutable reference, e.g. `serialize_query_result_cache`.

compiler/rustc_data_structures/src/fingerprint.rs

@@ -144,9 +144,8 @@ impl_stable_hash_via_hash!(Fingerprint);
 
 impl<E: rustc_serialize::Encoder> Encodable<E> for Fingerprint {
     #[inline]
-    fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        s.emit_raw_bytes(&self.to_le_bytes())?;
-        Ok(())
+    fn encode(&self, s: &mut E) {
+        s.emit_raw_bytes(&self.to_le_bytes());
     }
 }
 
@@ -187,10 +186,10 @@ impl std::fmt::Display for PackedFingerprint {
 
 impl<E: rustc_serialize::Encoder> Encodable<E> for PackedFingerprint {
     #[inline]
-    fn encode(&self, s: &mut E) -> Result<(), E::Error> {
+    fn encode(&self, s: &mut E) {
         // Copy to avoid taking reference to packed field.
         let copy = self.0;
-        copy.encode(s)
+        copy.encode(s);
     }
 }
 

compiler/rustc_data_structures/src/svh.rs

@@ -49,8 +49,8 @@ impl fmt::Display for Svh {
 }
 
 impl<S: Encoder> Encodable<S> for Svh {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        s.emit_u64(self.as_u64().to_le())
+    fn encode(&self, s: &mut S) {
+        s.emit_u64(self.as_u64().to_le());
     }
 }